Calcium silicate hydrate (C-S-H) is the binder in concrete, the most used synthetic material in the world. The main weakness of concrete is the lack of elasticity and poor flexural strength considerably limiting its potential, making reinforcing steel constructions necessary. Although the prop erties of C-S-H could be significantly improved in organic hybrids, the full potential of this approach could not be re ached because of the random C-S-H nanoplatelet structure. Taking inspiration from a sea urchin spine with highly ordere d nanoparticles in the biomineral mesocrystal, we report a bioinspired route toward a C-S-H mesocrystal with highly aligned C-S-H nanoplatelets interspaced with a polymeric binder. A material with a bending strength similar to nacre is obtained, outperforming all C-S-H – based materials known to date. This strategy could greatly benefit future cons truction processes because fracture toughness and elas- ticity of brittle cementitious materials can be largely enhanced on the nanoscale.